The problem is that the second you add the word "server" to a computer, the price doubles. For that reason it's usually cheaper to build your own (unless you get a great deal on a TS140).
Here is how I built my very cheap 6 core server.
(Note: I'm not going to explain how to put a computer together. If you've never done this, this blog post probably isn't for you)
First you need a case. I like the startlingly cheap yet not bad looking Rosewill FBM-02. You'll spend about 20 bucks for it. There's a picture of it up there.
Next you'll need a power supply. This is almost the most important part you'll buy. It needs to be reliable, and it needs to be power efficient since it will be running 24/7. I like this one:
Seasonic SS-300ESYou'll pay about 30 bucks for it. Very reliable. Very inexpensive. 300 Watts is all you'll need. That's more than enough for a small server.
Next you'll need to pick a processor. The best value proposition for something with lots of cores is the AMD FX-6300 Vishera Black Edition currently.
For aftermarket cooling, I'm a huge fan of the Arctic Alphine 64 GT. It's much better than the stock cooler, and very quiet. Loud servers suck.
Next you'll need a motherboard with a lot of options. The idea is that down the road if you want to toss in a video card/etc, you'll have that option. The main thing that matters to you for a server is as many ram slots as possible, and support for your CPU. I like this one:
The Gigabyte GA-78LMT-USB3 supports up to 32GB of ram. It's also wonderfully cheap and reliable. You can pick it up for about 40 bucks. It will easily run the FX-6300 at 4.1GHZ all day.
For those keeping count, we are just under 200 bucks. Which is pretty amazing considering we are almost done.
For memory, I'd start with 16GB. In the real world, for a server, you aren't going to see much difference between 1333mhz and 1600. So I'd get this:
Kingston Valueram 16GB 240 pin DDR3 (2x8) kit. Get either the 1333 or 1600. Whichever is cheaper. You'll pay about 130-140 bucks. You'll have the ability to add another 16GB later.
All that's left is highly variable in this market. You are looking for an OS SSD and a big data/media drive for content. The OS drive should be at minimum 100GB. The media drive should be the best deal you can get on huge storage. A quick spot check:
Crucial MX100 128GB - 60 bucks
Toshiba 3TB Sata 3.5" - 90 bucks
So for a grand total of about what you'd spend for an IBM Thinkserver TS140 with 4GB of ram and no drives, you'll have a six core 4.1Ghz machine with 16GB of ram, and 3TB of storage that can handle just about anything you throw at it while making no noise, and using very little electricity. But more importantly, you aren't locked into a certain hardware configuration. You'll be able to easily add more drives, upgrade the processor, add more ram, a video card if you want, more ethernet cards, etc. The way a server should be. You'll have something you can use for 5-10 years easy.
This is the setup I use for my Bitcoin/Plex/Sabnzbd/Sickbeard/Namecoin/devcoin/ixcoin/i0coin/Couchpotato/P2pool/Tekcoin/Transmission/NFS/Cifs/etc server. It's doing tons of shit. It isn't breaking a sweat.
Hope this helps